OBJECTIVE Leukemic hyperleukocytosis may cause organ- or life-threatening complications. Patients at highest risk appear to be those with acute myeloid leukemia (AML). Blast cell aggregation and thrombus formation in the microvasculature most commonly involves the central nervous system and the pulmonary circulation. We describe a child with AML and renal venous thrombosis (RVT), a previously unreported complication of hyperleukocytosis. METHODS A 17-month-old boy had a white blood cell count of 103 X 10(9) cells/L and RVT (hematuria, arterial systolic hypertension, unilateral nephromegaly, poor renal venous blood flow) at diagnosis of acute myelomonocytic leukemia (AML, FAB M4). CONCLUSIONS This case emphasizes the danger of hyperleukocytosis in AML and demonstrates that there may be other organ system dysfunction in addition to the well-described central nervous system and pulmonary complications. Renal venous thrombosis should be considered in the patient with leukemic hyperleukocytosis, hematuria, arterial hypertension, and appropriate radiographic findings. Aggressive cytoreductive measures should be pursued in such cases.